I demonstrate some flight issues with my Syma X5C quadcopter / drone and how to fix it. In this case I had a bent rotor shaft but also the motor was also not performing well.
I don't show too much detail about taking the quad apart because I did that in a previous video you can check out if you need that detail. It also shows how to replace motors. Have a look at https://www.youtube.com/watch?v=6hlfNd8NzWE